1. Field of the Invention
The present invention relates to steel products having improved grain size properties and machinability, and more particularly to steel products for carburizing and steel products for induction hardening and a process for producing the same.
2. Background Art
In general, steels for carburizing are subjected to hot working, cold working, machining and the like to form a predetermined product shape. Subsequently, the steels are carburized at 900° C. or above and quenched and are then tempered. In this case, however, in the carburizing, grains sometimes coarsen depending upon a combination of a former structure, carburizing conditions and the like. In order to prevent grain coarsening, for example, the content of aluminum (Al), niobium (Nb), nitrogen (N) or the like in the steel material is controlled to utilize precipitates of AlN, NbCN or the like as pinning particles. Thus, the prevention of coarsening of grains in the carburizing has been desired.
On the other hand, in automobile components and components for industrial machines, in many cases, medium-or high-carbon steels subjected to induction hardening and tempering have hitherto been used. Grain coarsening can be a main cause of deteriorated mechanical properties of these induction hardened components. In order to avoid the grain coarsening, it is important that, in the austenitizing, the temperature of the steel is not raised to an excessively high temperature. Nevertheless, in the induction hardening, a temperature gradient is likely to appear from the surface toward the inside of the steel because the steel should be heated in a short time. As a result, the temperature of a portion near the surface of the steel is likely to be higher than the other portions. In particular, when a relatively large case depth is contemplated, the temperature of the inside of the steel should also be raised to a value high enough to cause austenitizing. Therefore, the temperature of a portion near the surface of the steel becomes still higher. Thus, induction hardening can be said to be a heat treatment method which is likely to cause grain coarsening particularly in a portion near the surface of the steel. For this reason, also in the steel for induction hardening, the prevention of grain coarsening in the induction hardening has been desired.
Japanese Patent Laid-Open No. 283910/1996 discloses a steel material for induction hardening in which grain coarsening has been suppressed by finely precipitating TiN, AlN, niobium (Nb) carbonitride, and vanadium (V) carbonitride for pinning of grain boundaries. Since, however, these precipitates are not satisfactorily fine, satisfactory anti-grain-coarsening effect cannot be attained in the induction hardening.
Japanese Patent Laid-Open No. 80446/2000 discloses a titanium(Ti)-added steel for induction hardening. In this titanium(Ti)-added steel, not less than 0.05% of titanium has been added to a steel to precipitate a titanium compound and titanium carbonitride which can improve anti-grain-coarsening effect. This technique, however, does not aim at machinability. At the present time, any steel product for quenching having excellent machinability, which is an as-hot worked product and has not been subjected to an additional heat treatment such as annealing, is not known.